The Butcher's Nails were crude neural implants embedded in the brain, a technology of the fighting pits that traded sanity for savagery. They suppressed the centres of reason and empathy while flooding their bearer with rage and a narcotic reward for violence, until only killing could quiet the agony they inflicted.
The implants are inseparable from the tragedy of Angron, primarch of the World Eaters, who was fitted with them as a slave-gladiator on the world of Nuceria. The Nails warped his every thought and made peace a physical torment, and when he was reunited with his Legion he insisted his sons receive them too, remaking the proud War Hounds into a brotherhood of berserkers who lived and died by the red haze.
As the Age of Darkness ground on, the Nails ate ever deeper. They eroded discipline, memory, and self, leaving warriors who could barely recall their own names between engagements. In binding his Legion to his own suffering, Angron ensured the World Eaters would become an instrument of pure slaughter, offered at last wholesale to the Blood God. Few technologies in the annals of the war so perfectly married cruelty to purpose.